1. Field of the Invention
The present invention generally relates to acoustic surface wave apparatus and, more particularly, to the transducers used in those applications.
2. Description of the Prior Art
There are many systems in use today that encode data in surface acoustic waves. For example, in the field of nondestructive material evaluation acoustic waves are commonly propagated through various materials in order to detect manufacturing defects, cracks, and flaws. The defect or nonhomogeneous area in the material appears as a change or modulation of the acoustic waves. The position and size of the defect as indicated by the change is thus encoded into the acoustic waves. The defect in the material is discovered and evaluated by locating and measuring these changes. Data is also encoded into surface waves by acoustic imaging systems. In these systems the waves carry an acoustic picture of the object of interest and the encoded data contain information on the contrast of the picture.
In all of the systems where data are encoded into surface acoustic waves the waves are seldom processed to obtain a first and second order derivative information. Most commonly the data is displayed as received and no conversion or processing is performed. Any changes are noted by visually observing the signal as it is displayed on an oscilloscope.
If derivative information is required from surface acoustic waves, then an operational-amplifier differentiator is most commonly used. The surface acoustic waves are first converted into analog electrical signals and then an operational-amplifier network electrically performs the desired differentiation.
It should be noted that an operational-amplifier differentiator cannot directly convert the acoustic waves into derivative signals. In addition, such a differentiator requires subsequent demodulation in order to yield an output signal proportional to the derivative of the envelope of an input function. For example, for an amplitude modulated input signal A(t) sin .omega.t, the output of an operational-amplifier differentiator contains two terms A'(t) sin .omega.t + [A (t) .omega.] cos .omega.t. In order to obtain an output proportional to only the derivative of the envelope A'(t), further demodulation is required. Further, operational-amplifier differentiators fabricated from semiconductors have the problem of saturation and signal distortion when operated at high voltage levels. Also, it should be noted that present operational-amplifiers have a maximum operating frequency of approximately 100 MHz. This upper frequency threshold presents problems when high information storage density is required.